South Korea has twice the population and 40 times the economic might of North Korea. The South has a highly sophisticated military and a strong U.S. alliance. But North Korea is also one of the most militarized nations on Earth, bristling with nuclear and conventional arms that could inflict horrific damage on its southern neighbor, albeit at a suicidal cost.

North Korea's bombastic propaganda machine need only issue a missive threatening to turn Seoul into a "sea of fire" and the South Korean stock market takes a beating.

"It is difficult to keep North Korea under control despite the fact that the U.S. and South Korea are superior in military force, because they know that we won't use force first," said Park Syung-je, a military analyst at the Asia Strategy Institute in Seoul.

South Koreans have largely grown inured to the constant threat, but some acknowledge an underlying fear.

"North Koreans are scary. They are full of spite," said Kim Sung-ya, a well-dressed woman wearing bright lipstick and sunglasses who was strolling through a Seoul train station. "I'm afraid that they might fire something first."

On several occasions, most seriously in 1994, the U.S. contemplated a preemptive strike against the North Korean nuclear compound at Yongbyon, but quickly abandoned the idea. There is no doubt that if such a strike spun into a full-scale war, the United States could easily defeat North Korea — according to some estimates, in less than three days.

But victory would come at an unthinkable cost. Half of South Korea's 50 million people live within short-range striking distance of North Korea, and the thriving metropolis of Seoul is only 30 miles away. The North Korean capital, Pyongyang, is within 70 miles of the border. Both sides could expect horrific casualties.

North Korea does not have a workable nuclear weapon or intercontinental ballistic missile thus far, but it has the fifth-largest army in the world, more than 1.1 million soldiers under arms and 4.7 million on reserve, according to the International Institute for Strategic Studies. Most of the North's force is pointed like a dagger at South Korea. According to the London-based institute, it has 8,000 artillery systems and 2,000 tanks at the demilitarized zone on the border.

A computer simulation done in 1994, when the Clinton administration was contemplating airstrikes to take out the nuclear complex at Yongbyon, projected that if North Korea went all-out in retaliation, as many as a million South Koreans could be killed, as well as thousands of American troops stationed near the DMZ, according to a former South Korean official.

U.S. military planners use the phrase the "tyranny of proximity" to describe the geographic situation.

If the United States and South Korea launched a quick, all-out blitz against North Korea, there would be heavy casualties on the North Korea side. In a slower, more cautious campaign against military targets, losses would be greater south of the border, according to Park.

"Ultimately, North Korea is bound to lose, but millions of people would die," Park said.

Another scary scenario: A conventional war could quickly turn nuclear. In a much-discussed article that recently ran on the Foreign Affairs magazine website, scholars Keir A. Lieber and Daryl G. Press said that Kim Jong Un, fearing the same fate as other defeated leaders, such as Iraq's Saddam Hussein and Libya's Moammar Kadafi, could be tempted to use a crude nuclear weapon, delivering it through a commando raid or some other unconventional means.

"Nuclear weapons are really the weapons of the weak," said Lieber, an associate professor at Georgetown University. "If North Korea is facing conventional defeat, which seems likely given the superior might of the combined forces, they might not see any other option for avoiding regime change."

North Korea also deploys the craziness card to its advantage. The colorful bombast of its infamous propaganda mouthpiece, the Korean Central News Agency (threatening to "break the waists of the crazy enemies, totally cut their windpipes and thus clearly show them what a real war is like") is designed not merely to intimidate but to convince the outside world that it is dealing with lunatics.

And of course, when dealing with lunatics — as with suicide bombers — the whole premise of deterrence breaks down.